This disclosure relates generally to oil and gas well logging tools. More particularly, this disclosure relates tools for measuring rock formation properties such as density and porosity. This disclosure relates to an improved density and/or porosity tool having a targetless source of pulsed neutrons.
In petroleum and hydrocarbon production, it is desirable to know the porosity and density of the subterranean formation which contains the hydrocarbon reserves. Knowledge of porosity is essential in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of porosity is particularly useful in older oil wells where porosity information is either insufficient or nonexistent to determine the remaining in-place oil and to determine whether sufficient oil exists to justify applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas. Measurements using pulsed neutron generators are useful in determining porosity, hydrocarbon saturation, and hydrocarbon type. Pulsed neutron measurements may be used for determining formation Σ, porosity, density and elemental composition.
If the density of the formation is known, then porosity can be determined using known equations. A variety of tools exist which allow the density of the reservoir to be determined. Most of these tools are effective in determining the density (and hence porosity) of the reservoir when the borehole in which the tool is run is an uncased reservoir and the tool is able to contact the subterranean medium itself. However, once a borehole has been cased, there exists a layer of steel and concrete between the interior of the borehole where the tool is located and the formation itself. The borehole casing makes it difficult for signals to pass between the tool and the reservoir and vice-versa.
Many of the commonly used porosity and density measuring tools rely on the detection of gamma rays or neutrons resulting from activation of either a neutron source downhole or a gamma ray source. Existing logging tools and LWD design considerations rely on established source to detector distances or ratios of distances in the case of multiple detectors to provide various analyses related to the formation and borehole environment. A pulsed beam, partially or wholly of deuterium, is directed onto a suitable target having tritium and pulsed neutrons are emitted from the target.
There are several disadvantages to having a neutron emitting device including a target. These include:                Increased activation product associated with the target substrate material in the most immediate vicinity of the point of neutron generation        Degradation of output in time associated with burn through or sputtering of the target assembly's hydrogen occluder.        Degradation of output associated with contaminant sorptions of the target occluder is possible with a target material.        Scattering through the matrix material sharply reduces ion kinetic energy and event cross-section. So while the ion penetration depth is on the order of 0.1 μm, virtually all neutrons are produced at the very most beam-ward face of the target. While a sharply defined neutron plane of generation may be a good thing, there is a gross inefficiency inherent in this type of geometry or device due to the coulombic deceleration of the ions penetrating the target matrix material.It would be desirable to have a pulsed neutron source that does not include the target. The present disclosure satisfies this need.        